1. Field of the Invention
This invention relates to detonators for initiating explosive charges and, more particularly, to a detonator which is incapable of causing accidental bodily injury.
2. Description of the Prior Art
All commercial and military explosives require detonators to initiate them. The detonator is fired either by electrical impulse or by a burning fuse in order to generate sufficient heat to initiate the explosives in the detonator. The typical electric detonator consists of an electric bridge wire which heats very rapidly when a current passes through it, surrounded by a bead of pyrotechnic composition which is easily ignited. This bead is in turn coated with or in contact with a small layer of heat sensitive explosive, such as lead axide. The lead azide serves to initiate the main charge of high-strength explosive, which is also contained in the detonator. The main charge, typically PETN or RDX, is relatively insensitive to heat and shock. In the fuse-type detonator, a receptacle for the fuse replaces the two electrical leads and the bridge wire. Detonators measure approximately 1/4 inch in diameter and are 1/2 to 11/2 inches long, and have either aluminum or copper jackets.
The very thing that makes detonators useful causes them to be inherently dangerous. They convert a very small amount of input energy into an explosion of sufficient strength to initiate high explosives like dynamite. Detonators can be inadvertently set off by stray electromagnetic fields generated by radio transmitters, by shocks produced by vibrating the detonator or striking it with a hard object, by heat produced by even small fires and by many other types of mishandling. When the detonators explode, the high explosive contained therein is enough to remove a hand or do other serious injury or even cause death.
Attempts to enhance the safety characteristics of detonators have thus far been limited to improved electrical characteristics and changes in composition of the explosive load. Over the years, bridge wires which require a higher current have been adopted, thereby reducing the stray current hazard. Originally, mercury fulminate, a very dangerous material, was used as the heat sensitive explosive and the main charge. However, this explosive combination has now been replaced by lead azide and PETN or RDX combinations, which are less sensitive and thus less dangerous.
One relatively new approach which has increased the safety and reliability of commercial blasting operations has been the use of a detonating cord, such as PRIMACORD, to transfer the shock of a detonator to the main explosive blasting charge. PRIMACORD, a product of the Ensign-Beckford Company, Simsbury, Connecticut, has an explosive core of PETN enclosed by a wrapping of cloth and polyethylene plastic. The detonator is first used to initiate the PRIMACORD, and the explosive blasting charge is detonated when the detonation wave traveling through the PRIMACORD reaches the explosive blasting charge. PRIMACORD can also be used in place of individual electric detonators where a number of explosions are desired. Instead of placing an electric detonator on each explosive charge, a single detonator is used with a length of PRIMACORD extending to each explosive blasting charge. The PETN contained in the PRIMACORD core releases a significant amount of energy when it detonates, yet it is relatively insensitive and, when packaged in PRIMACORD, is virtually immune to accidental initiation by static electricity, stray currents or shocks. It is, therefore, considered by many experts to be safer than extensive networks of electric detonators, and the technique has gained widespread acceptance in the blasting industry.
Even the use of PRIMACORD in blasting operations has not totally eliminated the blasting cap and its inherent hazards, for PRIMACORD must itself be set off with a detonator. Usually, the detonator is taped to the end of the PRIMACORD or a special connector is used. Accidental initiation of the detonator during blasting preparations can cause severe bodily injury and can ignite the PRIMACORD, causing premature detonation of the explosive charge.